The invention relates generally to the field of electrical drives. More particularly, the invention relates to techniques for pre-charging a direct current (DC) bus in a power drive.
In the field of power electronic devices, a wide range of circuitry is known and currently available for converting, producing, and applying power to loads. Depending upon the application, motor drive systems may include circuitry that converts incoming power from one form to another as needed by the load. In a typical drive system, for example, a rectifier converts alternating current (AC) power (such as from a utility grid or generator) to direct current (DC) power. Inverter circuitry can then convert the DC signal into an AC signal of a particular frequency desired for driving a motor at a particular speed. The inverter circuitry typically includes several high power switches, such as insulated-gate bipolar transistors (IGBTs) that are controlled by drive circuitry. Motor drive systems also often include power conditioning circuitry, including capacitors and/or inductors, which remove undesirable ripple currents from its DC bus.
Sometimes during operation of a motor drive system, and particularly during start-up, high levels of in-rush current may be received by a motor drive in the motor drive system, which may cause various adverse affects to the motor drive. To avoid these high levels of in-rush current during start-up, a typical motor drive system may include pre-charge circuitry that applies a smaller initial current to the DC bus of the motor drive system prior to actually starting the motor drive. The pre-charge circuitry charges a number of capacitors coupled to the inverter before applying a full source voltage to the inverter. Such techniques may be referred to as pre-charging the DC bus.
Typical pre-charge techniques may include increasing a firing angle of semiconductor devices (e.g., thyristor) in a rectifier until capacitors on a DC bus are charged to some level or connecting a resistor with a contactor in parallel such that the resistor is bypassed via the contactor after the DC capacitors are charged. Other pre-charge circuitry may involve a three-way switch, which may connect the DC bus, pre-charge the DC bus, or disconnect the DC bus. However, such configurations may result in significant power loss through the diode and may not be used to isolate the drive from the common DC bus. Furthermore, each of the above described pre-charge circuit configurations and techniques involve using one or more circuit breakers to isolate or disconnect a drive from the common DC bus, which increases the overall size of each drive.